The former environment minister Peter Garrett underestimated the number of people who would take advantage of the money-making opportunity of the home insulation scheme and not uphold their duty of care to workers, the royal commission heard on Wednesday.

But he denied the scheme was "flooded with shonks".

The royal commission in Brisbane is examining the government processes and policies that set up the scheme, which has been linked to the deaths of four workers. On Wednesday Garrett reiterated statements he made the day before, taking responsibility for the scheme's rollout.

"I was a commonwealth minister, it was a commonwealth program," Garrett said. "I would never walk away from that."

But he said it was also a shared responsibility among other stakeholders and employers who had a duty of care.

"Before this scheme rolled out I think there were very few accidents in the insulation industry," said the commissioner, Ian Hanger QC, adding that problems occurred after an influx of people becoming installers, including a number of "shonks".

"The government can't regulate everything," Garrett said, agreeing with him.

"But you're right, the nature of the program … meant there were a lot of new entrants into the marketplace."

Some people who were reckless or "cavalier" took the opportunity, Garrett said.

Earlier on Wednesday Garrett conceded the government’s compliance and monitoring program could not keep up with the pace of insulation. More than 430,000 homes had been insulated just three months into the program.

"It seems to me that the great tragedy of the scheme was that these were young boys who hadn't had any workplace experience generally, so wouldn't have been able to say to a supervisor [if they had any concerns]."

Matthew Fuller was electrocuted while installing insulation in October 2009. He was the first of four men who would die while carrying out the work subsidised by the federal government's economic stimulus package.

"I think it's obvious that if you get a death in a program, it focuses everybody," Garrett said. "I was gutted when that happened. It meant a great deal of additional attention on safety necessarily took place."

Garrett was demoted from his portfolio in 2010 amid calls for his resignation after the scheme's rollout.

The commission continues in Brisbane. The former prime minister Kevin Rudd was expected to appear on Wednesday afternoon.